Generally, the present invention relates to ceramic composites and the preparation thereof, and more particularly to such composites in which single crystal silicon carbide whiskers are dispersed to provided improvements in the fracture toughness and fracture strength of the ceramic.
Recent emphasis has been placed upon the use of ceramic materials as structural components in heat engines and high-temperature conversion systems such as turbines. For the use of ceramic in such applications fracture toughness (K.sub.Ic) of the material is a critical consideration. Conventional ceramic materials have relatively low fracture toughness with the exception of Al.sub.2 O.sub.3 --ZrO.sub.2 and partially stabilized ZrO.sub.2. Monolithic ceramic material such as SiC, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) have a fracture toughness in the order of about 3 to 5 MPa.m.sup.1/2 and a fracture strength (.sigma..sub.f) in the range of about 30-100 ksi (210-700 MPa). Utilization of these ceramic materials for the fabrication of structural components for use in heat engines and other high-temperature conversion systems required the use of ceramic components with very small flaw size less than about 50 .mu.m) in order to provide acceptable fracture toughness. However, in structural components especially of complex configuration, the determination of such small flaw sizes has been very difficult to achieve by using nondestructive inspection techniques.
Efforts to overcome the lack of sufficient fracture toughness in ceramic material included the development of fiber-reinforced composites. For example, graphite fiber reinforced ceramics provided impressive fracture toughness and strength at ambient temperatures but these ceramic composites were found to be of questionable value when subjected to elevated temperatures because of the oxidation of the carbon fibers and the reaction between the carbon in the fibers and the constituents of the ceramic material. On the other hand, the use of inorganic fibers such as silicon carbide (SiC) filaments and chopped fibers for reinforcing or strengthening ceramic material exhibited some success but encountered several problems which considerably detracted from their use. For example, conventional silicon carbide filaments or chopped fibers are of a continuous polycrystalline structure and suffer considerable degradation due to grain growth at temperatures about about 1250.degree. C. which severely limited their use in high temperature fabrication processes such as hot-pressing for producing ceramic composites of nearly theoretical density. Further, during high pressure loadings such as encountered during hot pressing, the polycrystalline fibers undergo fragmentation which detracts from the reinforcing properties of the fibers in the ceramic composite. Also, these polycrystalline fibers provided insufficient resistance to cracking of the ceramic composite since the fibers extending across the crack line or fracture plane possess insufficient tensile strength to inhibit crack growth through the composite especially after the composite has been fabricated by being exposed to elevated pressures and temperatures as in hot pressing.